Architectural daylighting is the practice of placing windows http://en.wikipedia.org/wiki/Window or other openings and reflective surfaces so that, during the day, natural sunlight provides effective internal lighting within a building structure. Particular attention is given to daylighting while designing a building when the aim is to maximize visual comfort or to reduce energy use, for example, from the reduced use of artificial (electric) lighting. In this way, sunlight is used to reduce our dependence on convention (e.g., oil, coal and gas) energy sources.
FIGS. 13(A) and 13(B) are simplified cross-sectional side views showing a portion of a building 20 having a room 21 fitted with a light shelf 22, which is an example of an architectural daylighting element that allows daylight to penetrate into a building. Light shelf 22 is a horizontal light-reflecting overhang that is placed above eye-level and has a high-reflectance upper surface 23. Upper surface 23 is then used to reflect sunlight SL through an upper window 24 onto a ceiling 25 as deep into room 21 as possible. Ceiling 25 is formed is preferably formed by a light-scattering material that redirects the light downward for use within the room. Light shelves 22 are typically used in high-rise and low-rise office buildings, and in particular are disposed on the equator-facing side of the building, which is where maximum sunlight is found. Architectural light shelves have been proven to reduce the amount of artificial lighting in a building by reflecting sunlight deep into each room, thereby facilitating the reduction or possible elimination of incandescent and fluorescent lighting, depending on depth of the room.
A problem with conventional light shelves is that the effective lighting within a room changes significantly over the course of a year. FIG. 13(A) illustrates winter sunlight SL(t1), which is directed by the sun at an incident direction ID1 that forms a relatively shallow angle relative to light shelf 22. Due to the shallow incident angle, winter sunlight SL(t1) is directed through eye-level windows 26 and is reflected by high-reflectance upper surface 23 at a corresponding angle deep into room 21, thereby producing suitable daylighting substantially throughout room 21 during the winter months when the sun remains low on the horizon. In contrast, FIG. 13(B) illustrates summer sunlight SL(t2), which is directed by the sun at an incident direction ID2 that forms a relatively steep angle relative to light shelf 22. Due to the steep incident angle, summer sunlight SL(t2) is reflected by high-reflectance upper surface 23 at a corresponding angle only a short distance into room 21, thereby producing suitable daylighting at a distance D2 inside room 21. This migration of effective daylighting during the year can become inconvenient and possibly irritating to the inhabitants of room 21, and can significantly increase the need for incandescent/fluorescent lighting in deep regions 27 of room 21 during the summer months.
In addition to the recent increased use of sunlight to provide natural lighting, there is a current trend toward the production of large solar power stations that directly convert sunlight to electricity that is fed into the existing electrical grid, further reducing our dependence on conventional energy sources. For example, PV farms are solar-to-electricity power stations that utilize large numbers of solar photovoltaic (PV) cells to convert sunlight into electricity on a commercial scale. Many such PV farms having power production in the range of 40 MW to 60 MW have already been built, mainly in Europe and the United States, and PV farms having capacities of up to 1 GW or more are being proposed. As the amount of power generated by such PV farms increases, it will become more and more desirable to utilize technologies that increase each PV farm's output by even a few percent.
FIGS. 14 and 15 illustrate portions of a typical fixed-tilt PV farm 50, which represents one PV farm type that is currently being used for commercial solar power generation. Fixed-tilt PV farms are characterized by having flat-panel solar PV cells that are maintained in a fixed position that is inclined (tilted) relative to the ground in order to maximize the capture of sunlight. FIG. 14 is a top perspective view showing three exemplary PV panel assemblies 60-1, 60-2 and 60-3, where each panel assembly 60-1, 60-2 and 60-3 includes an associated group of solar cell panels 65-11 to 65-16, 65-21 to 65-26 and 65-31 to 65-36 that are maintained in a desired “fixed-tilt” arrangement. Each of the solar cell panels 65-11 to 65-16, 65-21 to 65-26 and 65-31 to 65-36 includes multiple flat panel solar cells 80 that are maintained in a flat (planar) arrangement by a suitable panel structure (e.g., a plate of glass to which solar cells 80 are attached). Panel assemblies 60-1, 60-2 and 60-3 are spaced at a predetermined offset spacing (pitch) P for reasons described below, and are maintained in the desired “fixed-tilt” arrangement by a corresponding support structure 70-1, 70-2 and 70-3. In the present example, each support structure (e.g., support structure 70-3) includes a base 71 that serves to support a panel support structure 72 at an inclined angle θ relative to substantially level ground G, where panel support structure 72 supports panels 65-31 to 65-36. For descriptive purposes, panel support structure 72 includes an upper (horizontal) edge 73, a lower edge 74, and opposing left and right side edges 75 and 76, although outer edges of panels 65-31 to 65-36 may define these structure edges in some embodiments. Solar cells 80 are thus supported by and maintained in a planar array by a corresponding panel support structure 72 such that solar cells 80 are disposed over substantially all of the available surface of each frame 72 (i.e., the area bounded by upper edge 73, lower edge 74 and side edges 75 and 76). On each panel assembly 60-1, 60-2 and 60-3, solar cells 80 are electrically series-connected according to known techniques.
FIG. 15 is a simplified side elevation view showing PV panels 60-1 and 60-2 of PV farm 50, showing situations at noon time of various days for simplicity. The tilt angle θ is selected according to the latitude of the installation, to optimize the total amount of sunlight intercepted during the year. Typical tilt angle values are in the range of 0.7 to 1.0 times the latitude at which the PV farm is installed. As mentioned above, most fixed-tilt PV farms are and will be installed in the mid term future in latitudes between 25 and 60 degrees away from the equator, since this coincides with geographical regions of good insulation providing power availability and strong economic activity creating power demand. This observation limits the analysis and design to this range of latitudes, thereby simplifying the task at hand. At these latitudes, the sunlight beams SLE are directed at a predictable angle at the vernal and autumnal equinox, and varies around this direction as shown between sunlight beams SLWS having a relatively shallow angle −Δ (=23.5 degrees) at the winter solstice, and sunlight beams SLSS having a relatively steep angle +Δ at the summer solstice. Note that the active surface R formed by solar cells 80 of each panel (e.g., panel 60-1) defines a planar surface A. By knowing the incident angle of sunlight during the course of a year, fixed inclined angle θ can be selected to maximize the total amount of solar radiation captured by solar cells 80 during the course of a year. For example, as indicated in FIG. 15, inclined angle θ of PV panel 60-2 may be set such that sunlight beams SLE are normal to the active surface of solar cells 80A (i.e., such that solar cells 80A are optimally positioned to convert received sunlight during the equinox periods) in order to maximize the solar radiation captured during a twelve month period.
FIG. 15 also illustrates the required offset spacing (pitch) P for PV farms constructed at a given latitude. As mentioned above, solar cells 80 of each PV panel 60-1 and 60-2 are series-connected, and therefore it is important for maximum total power generation by each panel that each solar cell 80 produces a substantially equal amount of power. In order for this to occur, each solar cell 80 must receive the same amount of sunlight, and shading of any of the solar cells must be avoided. In particular, as indicated in FIG. 15, at the winter solstice, sunlight beams SLWS must pass over upper edge 74-1 of solar panel 60-1 without casting a shadow on the lowermost solar cells 80A of panel 60-2. To achieve this homogenous illumination of all solar cells, solar panels 60-1 and 60-2 must be separated by offset spacing P.
Depending on latitude and exact solar farm layout, a yearly aggregate of between approximately 30 and 80 percent more sunlight is available within the footprint of a standard fixed-tilt PV farm than is actually intercepted by the photovoltaic panels. This is due to the conservative offset spacing P typically employed with electrically series-connected modules, which pose a strong requirement for homogenous illumination (i.e., as explained above, each solar cell 80 must receive substantially the same amount of light in order to maintain optical power generation). The extra sunlight would normally be absorbed unused in the space between the panels. For example, as indicated in FIG. 15, summer solstice sunlight beams SLSS1, SLSS2 and SLSS3 are indicated as passing directly onto ground G between panels 60-1 and 60-2, which illustrates a large amount of sunlight is not received by solar cells 80 during summer in order to provide homogenous illumination during winter.
What is needed is a low-cost sunlight redirecting element that can continue to redirect sunlight onto a fixed target even when the sunlight's incident direction changes over time, thereby facilitating the production of a low-cost, stationary solar systems (e.g., architectural daylighting systems and PV farms) for high northern or southern latitudes that efficiently utilize sunlight over a large portion of each year.